The specific aims of this proposal are two-fold: the first aim is to conduct a multi-center, randomized clinical trial to evaluate the efficacy of intervening to delay or prevent the onset of non-insulin dependent diabetes mellitus (NIDDM) in individuals at increased risk of NIDDM. These individuals will consist of persons with impaired glucose tolerance (IGT) and persons with a previous diagnosis of gestational diabetes mellitus (GDM) who now have IGT. The second aim is to conduct a multi-center, randomized clinical trial to evaluate the efficacy of intervening to prevent the deterioration of previously undiagnosed NIDDM (early, asymptomatic NIDDM found during screening). All subjects will be identified by screening. At least 50% of the sample will be minority. The individuals in the first group will be those with IGT. Individuals in the second group will be those with newly diagnosed NIDDM. Criteria will be set to take only mild NIDDM for the trial, the more severely compromised patients will be referred for treatment. Individuals in both groups will be randomized into an Experimental and a Control Group. Intervention for the Experimental Group will consist of two Levels. Level I will be initiated first and will be a diet, exercise, and behavior modification program. Level II, pharmacological intervention with metformin, will be added in Level I is unsuccessful. Intervention in the Control Group will only be contact every 6 months for glucose testing. Subjects will be followed for 5 yrs with periodic testing for glucose intolerance. The major end point will be the 2 hr post glucose venous plasma. value. Twenty sites will serve as Centers and each site will enroll 98 patients. Each site will emphasize one or two racial/ethnic groups. Our Center will plan to study Caucasian and Black volunteers, although we could also study Hispanics if the protocol writers wished us to do so. There will be an initial planning phase of 1 yr, a clinical trial phase of 5 yrs, and a close-out phase of 1 yr. There will be a Data Coordinating Center that will collect all data and will be in charge of the central clinical laboratory. Our Center has the appropriate population base for both the IGT and the GMD parts of this clinical trial and well-trained, experienced personnel for carrying out the protocol.